Overview
Because titanium (titanium and titanium alloys) has good mechanical and physical properties, its density is small and its strength is high, and the ratio of tensile strength σb to density ρ is σb/p=200, which is almost the highest among all metal materials. .
At the same time, it has very good corrosion resistance. The excellent chemical stability of titanium in strong corrosive environments and the strong self-passivation ability in electrolyte (water) make the application and promotion of titanium materials faster than many other metals.
Generally speaking, industrial pure titanium has better corrosion resistance than α-phase titanium alloys and β-phase or α+β-phase titanium alloys, and has a wider application range, although the strength is not as high as that of β-phase titanium alloys or α+β-phase titanium alloys. , but it has good plasticity and is easy to be processed and formed. Therefore, industrial pure titanium is the most commonly used material in titanium containers.
Selection principle
1) Deformed titanium materials should be supplied in the annealed state (M), and titanium castings should be supplied in the as-cast state.
2) TA3 in deformed industrial pure titanium TAO, TA1, TA2, TA3, due to its poor cold deformation ability, is generally not suitable for parts such as cylinder, head and blister, and can only be used for no cold deformation or cold deformation. smaller parts.
3) TA9 titanium-palladium alloy (Ti -0.2Pa) and TA10 titanium-nickel-molybdenum alloy (Ti -0.8Ni -0.3Mo) are mainly used in high temperature and wet chlorine-containing media and there may be crevice corrosion (especially TA9 is more resistant to crevice corrosion) It is especially suitable for parts such as tube sheets and flanges.
4) If there is a galvanic couple, usually the following measures can be taken:
①A kind of metal (usually the metal corroded by the galvanic couple) is covered with insulating material;
②Add a completely isolated insulating material between the two metals to avoid the formation of corrosion batteries;
③ Pull the distance between different metals, or change the position between them to avoid cathode pollution;
④ Avoid the formation of a large cathode and a small anode between the two metals to corrode the battery;
⑤ Adopt cathodic protection.
5) If there is crevice corrosion, the following measures can usually be taken:
① Adopt reasonable structural design, try to avoid or eliminate the phenomenon of gap retention and fouling, improve the flow state of fluid in the equipment, and avoid the formation of dead zone. When connecting internal bolts, try to use welded connections, and spot welding laps should use continuous as far as possible. Lap or butt welding.
②Apply palladium coating, oxidation or anodization to the surface where crevice corrosion may occur.
③ Putting putty mixed with NiO or nickel powder or MoO3 powder in the crevice can sometimes avoid crevice corrosion.
④Choose titanium materials that are more resistant to crevice corrosion, such as titanium-palladium alloy (TA9) or titanium-nickel-molybdenum alloy (TA10). These titanium materials are especially suitable for flanges with crevice corrosion on the flange sealing surface.
6) If there is hydrogen embrittlement rupture, usually the following measures can be taken:
①Choose titanium material with less hydrogen content.
②Prevent hydrogen absorption during processing and manufacturing, that is, avoid embedding iron particles on the titanium surface during processing and manufacturing such as cutting, stamping, coiling, welding, etc.; thermal processing and heat treatment must be carried out in a heating furnace with a slightly oxidative atmosphere; For some titanium equipment with complex structure, it is difficult to realize the welding joint with inert gas protection on the back side, and it is necessary to prevent contamination and hydrogen absorption during welding.
③Choose a suitable use environment: When used in an environment with dry hydrogen and wet hydrogen at a temperature of 71~316℃, if it contains a certain amount of oxygen and moisture, it can prevent hydrogen absorption. Titanium usually does not absorb hydrogen in oxidizing medium, neutral medium, weak reducing medium or reducing acid containing oxidant, or absorbs hydrogen very slowly; but when titanium surface is contaminated with iron, surface defects, When corrosion occurs locally or abnormal working conditions occur, hydrogen absorption embrittlement of titanium may occur. Titanium is prone to hydrogen absorption embrittlement in an environment where general corrosion or local corrosion occurs.
④By surface treatment, such as high temperature oxidation, anodization, etc., the resistance to hydrogen absorption can be improved.
⑤ Use corrosion-resistant alloys to improve the corrosion resistance of titanium and prevent hydrogen embrittlement of titanium.
7) Titanium materials are strictly prohibited to be used in liquid chlorine and dry chlorine applications.
8) Titanium materials are strictly prohibited to be used in fuming nitric acid with water content less than 2% or free nitrogen dioxide greater than 6%.
9) Titanium should be avoided in stress corrosion environment. Any medium with a tendency to stress corrosion, even if it is slightly corrosive to titanium, still has the risk of stress corrosion cracking, it is not suitable for use.
